The electroencephalogram (EEG) is essentially a waveform that is representative of electrical variations occurring between distinct locations on the human head. Characteristically, the EEG is a non-periodic, stochastic phenomenon. That is, oscillations in the electrical potential cannot be predicted, consequently only current and past information is available. The absence of recurring patterns in the waveform considerably complicates analysis of the EEG, as for use in diagnosis. However, in spite of the fact that the techniques are neither simple nor easy, neurologists have established principles and criteria for utilizing the EEG as an effective diagnostic tool.
In general, the EEG may be displayed either in a transitory manner (as on a cathode ray tube) or as a permanent record in the form of a strip chart. The cathode ray display of such waveforms is so transient that even an expert may have difficulty making an accurate analysis. As for a strip chart or paper record, the useful information can be extracted from the EEG only by recording at plotting speeds which result in a considerable volume of paper, e.g. 180 pages per hour. Essentially, the density of useful information in EEG records is low; and direct examination is laborious and time consuming.
A substantial part of the training of an EEG specialist consists of developing the ability to quickly recognize characteristic waves and patterns and to evaluate amplitude and frequency without the use of instruments. The demanded concentration for manually analyzing an EEG, along with other considerations, has resulted in various efforts to more efficiently decode and concentrate EEG information. In that regard, concurrent efforts have also been made to reduce the degree of expertise necessary for extracting useful information from the EEG. However, a need has long existed for a system to provide a more compact, simplified format for an EEG.
Over a considerable period of time, beginning in the 1930's and with no end in sight, several proposed methods of decoding and concentrating EEG information relied on Fourier analysis and primarily are referred to as "power spectra". Essentially, the basis of such techniques is the utilization of the Fourier transform to dissect the complex EEG waveforms on the basis of frequency into a plurality of component signals. The approach involves several objections, one of which relates to the inherent nature of the EEG as an aperiodic waveform.
The principal result of a Fourier-type analysis is to reveal and measure harmonics; however, there is no recognized significance of the higher order harmonics of the EEG. Also, the temporal sequence of the EEG events disappears as a result of the "averaging" operation which cannot be disassociated from the analysis in utilizing classical techniques. A single spectrum results from the analysis of several seconds of the EEG signal and (even if it represents correctly the total energy at a given frequency) it cannot show temporal distribution between waves or coincidence of typical elements. For instance, the simultaneous occurrence of spikes and slow waves in a classic "spike and wave" pattern is essential for the diagnosis of PM epilepsy. Harmonic analysis, however, even if it could signal the presence of spikes, could not ascertain their simultaneous occurrence with the slow wave.
Other examples can be found in characteristics of certain EEG waves known as "spindles", the EEG arousal, the K-complex, and so on, where the temporal sequence is significant. Single isolated events such as EEG spikes do not influence significantly the result of harmonic analysis and are not indicated in the power-frequency spectrum generally resulting from Fourier-type analysis.
In summary, it is the amplitude (voltage) of the EEG waveform (and not the energy or power content) which is significant for diagnosis and analysis. Short-duration waves (waveform components) of relatively large amplitude contain little energy but are highly significant.
In spite of the problems suggested above in analyzing EEG information by Fourier analysis techniques, substantial work has been done some of which is mentioned in a book, Monitoring in Anesthesia, edited by L. J. Saidman and N. T. Smith, published by John Wiley & Sons, Inc., 1978. The reference also refers to the work of the present development.
Considering the need for improved techniques in analyzing EEG information, an initial aspect involves devising a format for condensing the data represented by the aperiodic EEG waveform. In that regard, the present inventor initially devised a format to present EEG information in a concentrated form while preserving individual characteristics of the waves. The format was described in The Physiologist, Volume 18, No. 3, August 1975. In accordance with the format, the EEG is translated to a form which preserves the characteristics traditionally recognized by neurologists in analyzing EEG's. The waveform was considered as containing meaningful fluctuations defined either as spikes or certain waves classified in accordance with amplitude and duration as aperiodic transitory events. In accordance with the format of representation, the amplitude of waves and spikes is depicted as the height of an L-shaped character shown in a three-dimensional reference system where equivalent frequency is indicated on the horizontal and time is referenced on the depth axis. Utilizing such a format, it appeared that up to 180 seconds of EEG recording could be represented on a single page without loss of traditionally important information.
In general, the present invention is directed to a system whereby EEG information is decoded and presented in a concentrated format indicated above, so as to preserve the individual characteristics of the waves or waveform components. In that sense, the characteristics of the waveform which have been important for classic and traditional wave analysis are preserved in the display.